This invention relates to radar for space-borne use, especially to synthetic aperture radar (SAR).
SARs can be used to generate high resolution images of terrain, taking advantage of the relative velocity between the space-borne SAR and the ground below.
Particular features of the terrain e.g. crop distributions and characteristics can be highlighted by polarimetric SAR imaging. In this, the ground is illuminated by pulses of radiation which is plane polarised, and the energy scattered back towards the radar carries information about any essentially vertical or horizontal nature of the features illuminated. As a very schematic illustration, FIG. 1 shows a polarimetric SAR 1 in orbit around the Earth, emitting pulses 2 of vertically polarised radiation. These are scattered in all directions at the ground, and some are backscattered to the SAR. The scattered radiation, including the backscattered radiation, typically has horizontal as well as vertical components of polarisation.
A problem for such space-borne polarimetric SARs in the presence of the ionosphere 3, shown dotted in FIG. 1. This, in conjunction with the presence of the Earth""s magnetic field, causes a rotation of the polarisation of the plane polarised radiation on its path down to Earth, and another rotation on the way back to the SAR. This rotation is termed Faraday Rotation. While this is small for short wavelengths such as C-band (typically, 6 cm wavelength), it becomes sizeable in L-band (typically, 24 cm wavelength), and very large e.g. hundreds of degrees of rotation for P-band (typically 68 cm wavelength).
This causes errors in the interpretation of the information extracted from the SAR data, since if the radiation of the transmitted pulses is vertically polarised on leaving the SAR, the observed values of vertical and horizontal components of the radar returns at the SAR underestimate the true value of that vertical component and overestimate the horizontal component.
One known solution to this problem is to use circularly polarised radiation which is not affected by Faraday Rotation and, while it is popular for this reason in communications applications, it is less suited to the needs of radar remote sensing, because ground features in general possess inherent features that are vertical or horizontal rather than left or right hand helical, in nature.
Another known solution to this problem employs so-called quadrature polarised operation of the radar. Alternate transmitted pulses are vertically and horizontally polarised. The horizontal and vertical component of each radar return at the SAR is measured, and a knowledge of the relative phases and amplitudes of successive pairs of returns provides sufficient information for the amount of Faraday Rotation to be calculated and corrected for.
A simple example may help to illustrate how this is done. FIG. 2 shows a vertically polarised pulse 4 from the SAR impinging on a vertically-extending scattering surface 5 at the ground. At this first order reflection, some of which returns to the SAR, the scattered radiation remains vertically polarised.
Some of this vertically polarised scattered radiation undergoes second order reflection at an obliquely-extending scattering surface 6, which has the effect of producing horizontally and vertically polarised second order reflections. A horizontally polarised second order reflection 7 is illustrated in a direction back towards the SAR.
If the separation between first and second order reflections is random, as might be expected for scattering from any particular region of the Earth, (and assuming here that no Faraday Rotation takes place), the phase of the reflected horizontally polarised pulse 7 will be random relative to the reflected vertically polarised pulse.
It turns out that, in normal circumstances, this relation is generally true for no Faraday Rotation, and is termed azimuth isotropy. In other words, the mean correlation product between vertically and horizontally polarised returns (between co-polarised and cross-polarised returns more generally since this also applies if the incoming pulses are horizontally polarised), is zero.
In quadrature polarised data streams, the existence of four coherently related data streams from the radar corresponding to radiation emitted at the radar, scattered at the Earth""s surface and received back at the radar (VV, VHxe2x80x94co-polar and cross-polar returns from vertically polarised pulses, HH, HVxe2x80x94co-polar and cross-polar returns from the alternate horizontally polarised pulses) enable the data streams to be manipulated mathematically to represent data streams corresponding to a de-rotated incoming beam and de-rotated return beam, whose plane of polarisation has been rotated through an angle xcex8 relative to the initial frame of reference.
The set of four channels of received data is operated on successively, on an iterative basis, to determine the rotation angle corresponding to a minimum in correlation between the co-polarised and cross-polarised data streams (corresponding to the example of no Faraday Rotation of FIG. 2). This angle is then identified as the Faraday Rotation angle, and the calculated data streams corresponding to this angle of notional rotation are regarded as corrected data streams in which the contamination due to Faraday Rotation has been removed.
However, operation of a radar in SAR mode places specific SAR related criteria on its operation. Such criteria constrain the following parameters: the pulse repetition frequency (PRF) at which the radar must operate, the relationship between antenna area and the slant range to the region being imaged, incidence angle at which that region is viewed, RF carrier frequency at which the radar operates and orientation of the radar beam to the along track trajectory. A particular feature that fundamentally constrains the access that can be achieved to a given region is the maximum incidence at which acceptable SAR performance can be maintained. If attempts are made to operate the radar beyond this limit, the imagery produced by the system becomes unacceptably contaminated by responses from regions remote from the desired region. These artifacts are called ambiguities.
When the radar operates in the fully polarimetric, quadrature-polar mode with alternating pulses of H and V polar signals being emitted, the constraint on PRF for SAR operation has to be maintained for each sequence of transmissions. Thus the H sequence of transmissions must be at the same PRF as that for the V transmissions. Therefore the overall PRF at which the radar can operate is doubled.
This doubling of PRF in quadrature-polar mode causes the radar system to be more susceptible to ambiguities and further constrains the maximum incidence angle within which successful imaging can be conducted.
The invention provides radar for space-borne use, comprising means for transmitting pulses of radiation which are polarised in one plane, means for receiving radar returns, and means for adjusting the plane of polarisation of the transmitted pulses in dependence upon an estimate of the Faraday Rotation undergone by the plane of polarisation of the pulses.
Actual rather than notional pre-rotation of the plane of polarisation of pulses polarised in one plane, based on an estimate for the Faraday Rotation, enables correction to take place without recourse to doubling the pulse repetition frequency as is necessary if correction employing quadrature polarised data streams is used.
De-rotation of the returns may be performed in a number of ways, for example, in a notional manner by performing mathematical operations on the returns, or in an actual manner, for example, by rotation of the receiving antenna or rotation of its response pattern.
The amount of Faraday Rotation experienced by the propagating wave-front can be estimated in a number of ways.
It may be estimated by calculation from basic parameters if the total electron density within the plasma on the propagation path and the magnitude of the magnetic field present within that plasma are both known parameters. The total electron density may be established by calculation from measurements of the differences in round-trip time between the radar and the ground at different carrier frequencies.
The amount of rotation may alternatively be estimated by prediction from knowledge of the general spatial and temporal behaviour of ionospheric weather conditions.
Alternatively, the amount of rotation may be estimated by extrapolation from intermittent/spot measurements of Faraday Rotation or applicable ionospheric conditions, and predictions of ionospheric weather conditions.
Alternatively, the amount of rotation may be estimated by calculation from on-board observations of intermittent quadrature polar responses from the ground. While this technique is known in its application to quadrature polar data streams, its application as an intermittent measuring technique allowing dual polar observation to be made at significantly greater ranges is new. In other words, intermittent bursts of alternate horizontal and vertically polarised pulses could be emitted, to permit the Faraday Rotation to be calculated, so that the plane of the single plane of polarisation of the transmitted pulses, and those of the returns, could be appropriately adjusted.
As another alternative, the Faraday Rotation could be estimated by calculation from on-board observations of intermittent quadrature polar responses from the ground coupled with predictions of Faraday Rotational behaviour based on predictions of ionospheric weather conditions.
In a preferred embodiment, the means for receiving radar returns is arranged to receive radiation polarised in orthogonal planes, and the Faraday Rotation is estimated by an iterative combination of pre-rotation of the plane of polarisation of the transmitted signal and calculations based real-time analysis of the resulting orthogonal dual polar returns.
The output of each of these techniques is a signal whose magnitude is a measure of the angle through which Faraday Rotation will rotate the plane of polarisation of signals on the outward path and echoes on the return path.
A combination of means for estimating Faraday Rotation may be used to provide a better estimate of Faraday Rotation.